A biomarker is generally a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes or pharmacological responses to a therapeutic intervention. See Atkinson et al., 2001, Clin. Pharmacol. Ther. 69:89-95. Biomarkers vary widely in nature, ease of measurement and correlation with physiological states of interest. See, e.g., Frank et al., 2003, Nature Reviews Drug Discovery 2:566-580. It is widely believed that the development of new validated biomarkers will lead both to significant reductions in healthcare and drug development costs and to significant improvements in treatment for a wide variety of diseases and conditions. Thus, a great deal of effort has been directed to using new technologies to find new classes of biomarkers. See, e.g., Petricoin et al., 2002, Nature Reviews Drug Discovery, 1:683-695; and Sidransky, 2002, Nature Reviews Cancer 2:210-219.
The interactions of cell surface membrane components play crucial roles in transmitting extracellular signals to a cell in normal physiology and in disease conditions. In particular, many types of cell surface receptors undergo dimerization, oligomerization or clustering in connection with the transduction of an extracellular event or signal into a cellular response, such as, e.g., proliferation, increased or decreased gene expression or the like. See, e.g., George et al., 2002, Nature Reviews Drug Discovery 1:808-820; Mellado et al, 2001, Ann. Rev. Immunol. 19:397-421; Schlessinger, 2000, Cell 103:211-225; and Yarden, 2001, Eur. J. Cancer 37:S3-S8. The role of such events in diseases, such as cancer, has been the object of intense research and has led to the development of several new drugs and drug candidates. See, e.g., Herbst and Shin, 2002, Cancer 94:1593-1611; Yarden and Sliwkowski, 2001, Nature Reviews Molecular Cell Biology 2:127-137; McCormick, 1999, Trends in Cell Biology 9:53-56 (1999); and Blume-Jensen and Hunter, 2001, Nature 411:355-365.
Expression levels of individual cell surface receptors, such as Her-2 in breast cancer, have been used as biomarkers, especially to determine patient prognosis or whether a patient will or will not respond to certain treatments. Conventional immunohistochemical (IHC) or fluorescence in situ hybridization (FISH) analyses have been used to detect Her-2 overexpression to determine whether treatment with a Her2-acting agent, e.g., trastuzumab, is warranted. Unfortunately, IHC and FISH have certain limitations as diagnostic tools in that they are not necessarily accurate and also prone to different interpretations by different laboratory personnel. Her-2 is also over-expressed in other cancers such as ovarian cancer, non-small cell lung cancer, colon cancer, prostate cancer and pancreatic cancer. See Mosession et al., 2004, Semin. Cancer. Biol. 14:262-270.
A subgroup of Her-2-overexpressing tumors also have p95Her-2 (p95), an N-terminal truncated version of Her-2 that has shed the ectodomain, to which trastuzumab binds. Data suggest that the presence of p95 correlates to the extent of lymph node involvement, suggesting that p95 may be an important prognostic factor for breast cancer metastases. See Molina et al., 2002, Clin. Can. Res. 8:347-353. Interestingly, trastuzumab binds Her-2 but cannot bind the p95 truncated Her-2 so trastuzumab is ineffective in patients with high levels of p95.